1. Field of the Invention
The present invention relates to a tandem-type image forming apparatus including a plurality of image forming units and an intermediate transfer belt.
2. Description of the Related Art
An electrophotographic image forming unit used in a currently widely used image forming apparatus, such as a copier and a printer, includes a multitude of components, such as a photoconductor serving as an image carrier, a charging device, a development device, a cleaning blade forming a cleaning device, and a lubricant application device for extending the life of the photoconductor and ensuring the cleaning performance. The lubricant application device includes, for example, a solid lubricant, a lubricant application brush, and an applicator blade for uniformly applying the lubricant. Further, a tandem-type color image forming apparatus is widely used which includes a plurality of image forming units for a plurality of colors and an intermediate transfer belt.
In recent years, along with the spread of color image forming apparatuses, demand for a reduction in cost and size of the image forming units has only increased. To reduce the cost and size of the image forming units, it is effective to simply configure each of the image forming units to include only a photoconductor, a charging device, a development device, and a cleaning device, with omission of the lubricant application device.
Further, in an environment in which the image forming apparatus is commonly used, such as an office, the number of outputs of monochrome images is usually larger than the number of outputs of color images. That is, in the image forming apparatus used in an environment such as an office, a black image forming unit for the black color is usually used more often and therefore has a shorter lifespan than that of color image forming units for yellow, cyan, and magenta. This is because the photoconductor in the black image forming unit for the black color is operated more frequently and thus longer than the photoconductors in the color image forming units for the yellow, cyan, and magenta, and thus the photoconductor for the color black is abraded faster in the black image forming unit than in the color image forming units.
To address the faster abrasion and consequent shorter lifespan of the photoconductor in the black image forming unit than in the color image forming units, various proposals have been made. For example, in one approach involving a direct-transfer tandem-type image forming apparatus including a plurality of photoconductors and capable of forming a color image, a black image forming unit provided on the extreme downstream side in the direction of rotation of a transfer feed belt includes a charging device (i.e., charger) configured as a non-contact corotron charger, with each of the color image forming units including a charging device configured as a contact-type charging roller (i.e., charging roll). With this configuration of the image forming units, the abrasion of the photoconductor due to friction between the photoconductor and the charging roller in contact therewith is minimized and the lifespan of the photoconductor for the black color is made to last longer than in a configuration in which the charging devices for the respective colors are all configured as contact-type charging rollers.
Further, the application of a lubricant to a photoconductor is known to extend the life of the photoconductor. Therefore, providing a lubricant application device to the black image forming unit is also effective in extending the life of the black image forming unit. That is, the application of a lubricant to the photoconductor for the black color reduces the abrasion of the photoconductor for the black color, and consequently extends the life of the black image forming unit.
In addition, in recent years, an intermediate transfer method using an intermediate transfer belt has been commonly employed in tandem-type color image forming apparatuses. The black image forming unit is typically provided on the extreme downstream side in the direction of rotation of the intermediate transfer belt for such reasons as reducing the time taken for the first print. In this case, however, the lubricant applied to the photoconductor for the black color is affected and degraded by the charging device, and a part of the degraded lubricant is transferred from the photoconductor onto the intermediate transfer belt in a primary transfer section of the black image forming unit. Thereafter, in a primary transfer section of the image forming unit located on the extreme upstream side in the moving direction of the intermediate transfer belt, a part of the lubricant transferred to the intermediate transfer belt is transferred onto the photoconductor of the extreme upstream image forming unit.
If the charging device of the extreme upstream image forming unit is a contact-type charging roller, as in the above-described background image forming apparatus, the lubricant tends to adhere to the charging roller via the photoconductor and contaminate the charging roller. If the degraded lubricant thus adheres to and contaminates the charging roller, the photoconductor of the extreme upstream image forming unit may fail to be favorably and uniformly charged, and such a failure may result in the generation of an abnormal image.